A leaking flat roof is one of the most common calls we get across Wandsworth and the wider West London area. It’s almost never as simple as “the roof is old.” There’s usually a specific cause, and finding it is most of the job. Once you know where the water is getting in, the fix itself is often straightforward.
Failed joints and seams
Older felt roofs are built up from overlapping sheets. So the joints between those sheets are the weakest point in the whole system. As felt ages, the bitumen bonding the layers together dries out and loses its seal. The joint eventually lifts or splits. Also, water can travel sideways along the roof deck before it finds a way through the ceiling below. That’s why the drip inside the house often appears some distance from the actual fault. Tracing the leak from the joint pattern outside gets a faster result than guessing from the stain inside.
Blisters and splits in the membrane
Trapped moisture under a felt roof expands in the heat and contracts in the cold. Over enough cycles, that movement raises a blister in the surface. Once a blister splits, and it eventually does, it becomes a direct route for water into the roof structure. However, GRP and EPDM roofs are far less prone to this, since they’re installed as a continuous membrane rather than overlapping sheets. That’s one of the main reasons we usually recommend moving away from felt when a flat roof needs full replacement, not just a patch.
Ponding water
Flat roofs aren’t actually flat. A well-built one has a slight fall, usually around 1 in 40, so rainwater drains towards an outlet rather than sitting on the surface. Where that fall has been lost — through age, structural movement, or a poor original installation — water collects in puddles that can take days to evaporate. Standing water accelerates wear on any covering. It’s a strong sign the roof needs more than a spot repair, and the structure or insulation build-up beneath may need adjusting to restore proper drainage.
Perished flashing at the upstands
Every point where a flat roof meets a wall, chimney or parapet needs flashing to bridge the join and keep water out. This detail fails more often than the flat surface itself. Lead flashing that’s come loose, cracked mastic, and a poorly dressed upstand are all common culprits. Because the fault sits right at a junction, it’s often mistaken for a problem with the wall above rather than the roof. We check every upstand and abutment as standard on every flat roof leak call-out.
Should you repair or replace?
A targeted repair is usually the sensible option if the leak is isolated to one joint or section of flashing, and the rest of the covering is sound. But multiple blisters, widespread ponding, or repeated leaks in different places usually mean the covering has reached the end of its working life. Repeated patch repairs cost more over time than a single re-covering in GRP or EPDM. We cover this decision in more depth in our guide to repairing versus replacing a roof, which applies just as much to flat roofs as pitched ones.
What we check on a Wandsworth call-out
A typical Wandsworth extension roof is 15–25 years old. That puts a lot of them right in the window where felt starts to fail, but before the wider roof structure is affected. On every leak call-out, we check the joints, the flashing, the fall of the roof and the state of any outlets before recommending a fix. It’s the same process whether the property is in Wandsworth, Chelsea, or Fulham.
Got a leaking flat roof? Get in touch and we’ll inspect it and give you an honest repair-or-replace recommendation, not just a quote for whichever is more profitable.